Your MPG sounds inline with what I've seen from 2 different motorhomes with the 12 valve 8.3l in them. I logged quite a few miles in one, and weight didn't really seem to matter, speed wasn't a huge factor either, you could count on 8 1/2 out of it in the mountains, on the flat, towing, or empty. My neighbor has a 32 or 34 foot pusher with one in it, and he is right at the same no matter how he drives it. That said, I know somebody with an 8.3l ISC, and he said he's seen 11-12 out of it.

Glad you are back on the road making deliveries. Hope you can get the MPG up where you want it.

What was the milage in the 3500 loaded and unloaded vs the 8.3?

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Silver Birch Metallic 2004.5 GMC K2500HD with 265/75's
Banks super scoop, tubing, intercooler, and cat back with the IQ on the Window (for now). BD driver side Exhaust manifold.
Airdog 150 on frame rail. Amsoil synthetics for lubrication, and dual Amsoil filters on the driver frame rail. PPE centrifuge install.
EGR blocked. PCV rerouted. Resonator blocked (reinstalled for looks). Bladerunner installed. Dead kitty by MBRP (front pipe) and down tube replaced with the MBRP (heat wrapped from the turbo down to the front pipe).
Performax Gauges by ISSPRO in pod over mirror and overhead console (console pod by PPE). Mike L Transmission.
----Looking to do an onboard air with a set of truck air horns. Want Nathan K5LA's but not in the budget at the moment.----

4 fillups so far... 1-3 tanks at low 8mpg, #4 at low/mid 10mpg.. kinda baffled by that see how it plays out..
Measured the frame hieghts over axle.. front 36" rear 33" going to have to fix that
Second load picked up/Delivered

As for the frame loaded vs unloaded, are you running rear air suspension? Nice looking setup. Any plans on painting it? PPG used to make a color called Lemon Ice. One of the brightest yellows I have ever seen. Guy I worked for did a custom paint job for his brother and mixed it with Hot Lick Hot Red. Total number of paint mixes was 9. Mixed one inch of paint total each time. Fist one solid yellow where he started on the roof. Next one 7/8" yellow, 1/8" red and made 2 passes around the truck starting the fade. Last one was solid red. Add a pearl flame job to the front and that would be one HOT looking HOT SHOT truck.

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Silver Birch Metallic 2004.5 GMC K2500HD with 265/75's
Banks super scoop, tubing, intercooler, and cat back with the IQ on the Window (for now). BD driver side Exhaust manifold.
Airdog 150 on frame rail. Amsoil synthetics for lubrication, and dual Amsoil filters on the driver frame rail. PPE centrifuge install.
EGR blocked. PCV rerouted. Resonator blocked (reinstalled for looks). Bladerunner installed. Dead kitty by MBRP (front pipe) and down tube replaced with the MBRP (heat wrapped from the turbo down to the front pipe).
Performax Gauges by ISSPRO in pod over mirror and overhead console (console pod by PPE). Mike L Transmission.
----Looking to do an onboard air with a set of truck air horns. Want Nathan K5LA's but not in the budget at the moment.----